Abstract:

This thesis explores the nature of the subcontinental lithosphere underlying the southwestern margin of the Sao Francisco craton and the relation of variations in the petrochemistry of kimberlites and related alkali igneous rocks to variations in age, thickness and thermodynamic history of their continental lithospheric hosts. The Sao Francisco craton is a mid- to late-Archean basement granite-greenstone terrain flanked to the west by the Proterozoic Tocantins Province (Almeida, 1977; Almeida et al, 1981). New Rb-Sr and Sm-Nd data are presented for both on- and off-craton crustal rock sequences. The ultramafic greenstone association of the Rio das Velhas Supergroup yields 3.2 Ga Rb-Sr and Sm-Nd ages, in agreement with Widespread 3.2 Ga old zircons from the area. Granitic gneiss and juvenile granitoids associated with the greenstones in the Congonhas area give a Transamazonian 2 128 Ma Rb-Sr age, which is in agreement with a published 2 124 Ma zircon age. Further west, syntectonic granitoids and metabasalts from the Araxa Group define a 711 Ma Rb-Sr isochron. This latter age is interpreted as a Sr-isotope re-homogenisation related to the development of the Brasilia orogenic and foreland thrust belt. A 823 Ma Sm-Nd errorchron indicate that these rocks may be coeval to the felsic volcanism of the Araxa Group which was recently dated at 794 Ma by zircon work (Pimentel et al., 1991). Further to the west still, combined samples from the Niquelandia mafic-ultramafic igneous complex and associated granitic basement rocks ),yield a 1.26 Ga Rb-Sr isochron-on, which is best interpreted as a metamorphic age. Crystallisation ages of the crustal sequences decrease and [epsilon]<sub>Nd</sub> values increase with increasing distance westward from the Archean craton margin. The isotopic characteristics are consistent with a model which requires that large volumes of crust, derived in the Proterozoic from mantle reservoirs similar to the sources for modern oceanic basalts, were accreted onto the pre-existing Archean nucleus during the Brasiliano orogenic event. The Proterozoic rocks which overly and flank the Sao Francisco craton margin are intruded by Cretaceous kimberlites, olivine melilitites, tuffaceous diatremes and carbonatite complexes. Eight of the freshest representatives of the alkaline magmatism are described in terms of their age and mode of emplacement, petrography and whole-rock geochemistry. Kimberlites and kimberlite-related rocks have compositions similar to that of primary liquids derived from garnet peridotites. The kimberlite magmas are suggested to have resulted from entrainment of enriched lithosphere in plume-derived small-volume melts. The source character of the kimberlitic rocks is similar to that of carbonatites and other alkalic volcanics in the area, but is dissimilar to that of kimberlites elsewhere in the world. Compared to the source of micaceous Group II kimberlites, both the isotopically transitional kimberlites from southern Africa and the Brazilian kimberlites originated from sources with lower time-averaged Rb/Sr and Nd/Sm. Compared to the other rock types investigated, the lower time-averaged Rb/Sr, Nd/Sm and Pb/U ratios of the Brazilian kimberlites might be related to small but significant amounts of a distinctive high <sup>238</sup>U/<sup>204</sup>Pb (HIMU) component in these kimberlites. Major and trace elements of the alkalic rocks change with petrographic character towards more evolved compositions along liquid evolution paths indicative of shallow-level, olivine-dominated crystal fractionation. A restricted range of isotopic signatures, and the absence of any correlation between <sup>87</sup>Sr/<sup>86</sup>Sr and 1/Sr, suggest that the shallower alkalic rocks were probably derived by melting of a light-REE enriched lithospheric mantle source rather than through crustal contamination of asthenospheric melts. Compared to the kimberlites, the other alkalic rocks studied have a greater lithospheric component. The Nd isotope characteristics of such inferred lithospheric component are compatible with an origin closely related to the evolution of the Proterozoic rocks of the Tocantins Province. The average isotope composition of the studied rift-related volcanics may be taken as an approximation of the "Enriched Mantle I" (EMI) component previously identified as an extreme composition in the Walvis ridge basalts (Richardson et al., 1982). The source of the alkaline occurrences, the source of the high-Ti basalts of the northern Parana Basin, and the source of some Ocean Island Basalts (OIB) with Dupal signatures in the South Atlantic (viz. the Walvis Ridge basalts) are all isotopically similar to each other and closely related to this EMI-like component. The variation in isotope composition of the Brazilian kimberlites and shallower derived alkalic rocks is proposed to represent mixing between variable amounts of deep-derived HIMU-like components and shallower, volumetrically dominant, EMI-like lithospheric mantle materials. The HIMU-, and EMI-like signatures in particular, are envisaged to be concentrated in laterally extensive but vertically distinctive portions of the sub-continental mantle beneath the SW Sao Francisco craton margin. Isotope variations are complemented by a distinct range of Platinum Group Element (PGE) signatures in the studied rocks. The overall geochemical variation can be modelled by a process of physical assimilation and dilution of a PGE-rich material from the EMI-like upper mantle lithosphere by PGE-poor small volume melts with HIMU-like characteristics. The geochemical similarities of the source of the Mesozoic Minas Gerais alkalic volcanics to the source of the HTZ Parana basalts emplaced along the craton margin are credited to enriched to EMI-like lithospheric mantle materials which were involved in the generation of both rock sequences and, to a lesser extent, in the generation of the Minas Gerais kimberlites. Whilst the isotope evidence provide further evidence that discrete large-scale geochemical domains existed in Southern Gondwana, it is here suggested that such domains were not necessarily related to ancient lithospheric chemical heterogeneities or bound by ancient structural features, but rather that these domains reflect mixing processes that can be ascribed to specific geodynamic mechanisms. The similarity of the inferred source of the studied occurrences to the source of some OIB with Dupal signatures in the South Atlantic is ascribed to mantle processes during which overthickened Brazilian lithosphere was delaminated and contaminated a belt of South Atlantic asthenosphere which is now erupting at hotspot islands and which, in turn, may contaminate nearby sections of the Mid-Atlantic ridge. Peaks of Mesozoic magmatism along the south-western margin of the Sao Francisco craton were, contemporaneous with changes in plate vectors during the opening of the South Atlantic. It is suggested that changes in within-plate stress fields and associated fault reactivation along. old lithospheric shear zones were the main control of sites of intraplate alkaline magmatism such as that along the south-western Sao Francisco craton margin.